1. Field of the Invention
The present invention relates to glass or glass material for making rigid disk substrates and rigid disks substrates made therefrom.
2. Prior Art
Glass has the advantages, among others, that it has less surface roughness and is planar in comparison to metals, such as aluminum or metal alloys, and thus is used as a substrate for a data recording medium or carrier (rigid disk). These substrate glasses must be able to stand increased chemical, thermal and mechanical loads and stresses in use. Thus they experience high temperatures with rapid cooling rates during coating (for example by cathode sputtering). High mechanical loads occur when they are used as rigid disks, e.g. building up clamping stresses on a rotation axis of up to 100 N/mm.sup.2 and additional stresses by centrifugal forces in operation at high rotation speeds of currently 3,500 to 10,000 rev per minute. These loads can only be withstood by 0.25 to 3.0 mm thin glasses when they are pre-stressed. Since the increase of the mechanical load carrying ability by means of thermal pre-stressing is limited to glasses with a minimum thickness of 3 mm, glasses must be chemically pre-stressed for the above-described purpose. Significantly they are pre-stressed by ion exchange in a salt bath under the transformation temperature T.sub.g, i.e. they have sufficient suitable ions such as Li.sup.+ and/or Na.sup.+ ions for the exchange. Besides the surface planarity the chemical resistance of the substrate glass is of significance for their function as rigid disks, when the write-read head slides on an air cushion over the rotating rigid disk with a spacing of currently about 50 nm. This spacing must be maintained for an unobjectionable operation. However it is reduced when the surface of the fixed disk substrate is unstable to atmospheric influences and a chemical attack on the surface makes the surface rough prior to coating or when the surface looses its adherence properties for the applied coating sequence because of atmospheric influences and it loosens because of that. Thermal expansion properties of the glasses used for making rigid disk substrates are also important and should not be too different from those of the coating material (e.g. co-alloys with thermal expansion coefficients (.alpha..sub.20/300 .gtoreq.12.times.10.sup.-6 /K) and above all not to different from that of the clamping material and spindle material of the operating mechanism (with .alpha..sub.20/300 .gtoreq.12.times.10.sup.-6 /K), in order to avoid stresses and strains.
The lowering of the glide height of the read-write head over the rigid disk is a prerequisite for increasing the information density and read/write speed. A reduced travel or slide height allows an increased write density and a higher rotation speed of the fixed disk.
The travel/slide height cannot be arbitrarily reduced, because of fluctuations in the drive system during the rotation of the rigid disk, which are excited by strong location variations in air currents or turbulence and thrust fluctuations which express themselves in a sort of fluttering motion of the rigid disk. When the travel/slid height of the read/write head is to be reduced, these deviations from the rest position lead to a loss of correlation of the read/write head to the information content of the spots on the rigid disk ("runout") or it also to a collision with the fixed disk ("head crash").
In order to avoid this and to permit a rotation speed of more than 10,000 rev/min, the rigid disk needs a high shape stability, which currently glasses and glass-ceramics have not been able to provide for glasses used for rigid disks.
A composite material composed of Al-B-C is known as a material for these high rotation speeds (IDEMA, Alternative Substrates III (San Jose, Calif., Sep. 5, 1995), pp. 55 to 60, D. J. Perettie, et al, "The Alternate Alternative Substrate --"Chemically Strengthened" Aluminum"). This composite material has low density, a high strength and a very high specific elasticity modulus E/.rho.. The stiffness of a rigid disk, i.e. the resistance to bending, is proportional to (E/.rho.)*d.sup.3, wherein d is the thickness of the rigid disk. The above-mentioned material may be polished to the required surface quality with a roughness value (RMS-average) of less than 0.4 nm however only with great effort. Above all the making of rigid disks from this material is very expensive because of its great abrasion resistance.
A composite disk made from glass and a viscoelastic material, in which the fluctuations are damped by the viscoelastic material, for example plastic material, such as synthetic rubber, or a polyester, polyurethane or polyamide, is described in WO 96/04651. Disadvantageously the disk made in this manner is very expensive and the viscoelastic material becomes fatigued (embrittled) after some time and then can no longer operate as a fluctuation damping device. Furthermore the plastic material used can out-gas, when the magnetic coating is deposited at higher substrate temperatures by cathode sputtering, and because of that the quality of the applied coating is impaired.